Studying removal of anionic dye by prepared highly adsorbent surface hydrogel nanocomposite as an applicable for aqueous solution

In this study, a Sodium alginate-g-poly (acrylamide-clay)/TiO2 hydrogel nanocomposite [SA-g-p(AM-Bn)/TiO2] was synthesized using the biopolymer sodium alginate (SA), acrylamide (AM), and bentonite clay (Bn) as hybrid materials embedded with titanium dioxide nanoparticles (TiO2NPs) for the removal of toxic Congo Red (CR) dye from an aqueous solution. The [SA-g-p(AM-Bn)/TiO2] nanocomposite has been described on the basis of thermal stability, morphological analysis, estimation of functional group, and crystalline/amorphous character by TGA, EFSEM/EDX, TEM, FT-IR, and XRD analysis, respectively. The effects of operational parameters toward the CR dye adsorption on [SA-g-p(AM-Bn)/TiO2], including contact time, adsorbent dosage, initial concentration, initial pH, and temperature were investigated. The maximum adsorption efficiency was found to be 185.12 mg/g for [SA-g-p(AM-Bn)/TiO2] in 100 mg/L of solution CR at pH 6.0 within 1 h. The equilibrium isotherms, kinetics, and thermodynamics parameters of adsorption were examined, and results showed that the isotherm fitted the Freundlich model and the kinetics adsorption model of CR followed pseudo-first-order, thus indicating physisorption of anionic-CR onto the sorbent due to the development of an electrostatic attraction bond. Thermodynamic parameters for [SA-g-p(AM-Bn)/TiO2] have values (ΔG and ΔH) reflecting the spontaneous and endothermic nature of the adsorption processes. Moreover, [SA-g-p(AM-Bn)/TiO2] presented outstanding excellent reusability and recyclability with a relatively best removal percentage as compared to [SA-g-p(AM-Bn)] and suggested their applicability towards the textile industry and water purification purposes.


Preparation of the TiO 2 NPs
Titanium dioxide nanoparticles (TiO 2 NPs) were prepared by thermal hydrolysis of titanium (IV) bis(ammonium lactate) dihydroxide (TALH) in a stainless steel autoclave.10 mL of TALH solution was mixed with 100 mL of distilled water and 0.1 M ammonium hydroxide (NH 4 OH) in a 250 mL Teflon cup.The resulting mixture was stirred for 10 min to ensure thorough mixing.The Teflon cup was then sealed in the autoclave and placed in an electric furnace at 160 °C for 24 h.After thermal hydrolysis, the autoclave was cooled to room temperature, and the TiO 2 NP powder was collected by high-speed centrifugation (at least three cycles) and washed with deionized water (at least four times).Finally, the TiO 2 NPs were dried overnight in an oven at 60 °C35 .
A sodium alginate-grafted -grafted-poly (acrylamide-bentonite clay) TiO 2 hydrogel nanocomposite SA-g-P(AM-BC)/TiO 2 nanocomposite was synthesized.Different amounts of SA (1-4) g were dissolved in 150 mL of deionized water and added and stirred for 2 h at 45 °C.Variable amounts of acrylamide (AM) monomer (2-5) g were dissolved in 50 mL of deionized water.Then, variable amounts of bentonite clay (0.5-2) g were dissolved in 50 mL of deionized water and stirred for 1 h.The prepared bentonite clay dispersion was then gradually added to the SA and AM solution mixture and further stirred at 25 °C for 2 h to obtain a homogeneous dispersion gel.In the second step, surface functionalization with TiO 2 nanoparticles, the polymer matrix dispersion prepared in step 1 was added dropwise using a syringe needle to a bath solution composed of 0.5 w/v% TiO 2 nanoparticles and 4 w/v% CaCl 2 .2H2OThis simultaneous ionic crosslinking with Ca 2+ ions and surface impregnation with TiO 2 nanoparticles yielded the final biopolymer nanocomposite.Note: The best weight ratios of biopolymer were (4 g) SA, (2 g) AM and Bn clay (2 g), as the resulting beads were able to swelling to a very large compared to the rest of the preparation ratios, as shown in Fig. 1.

Adsorption studies
Congo red (CR) dye adsorption onto SA-g-p(AM-Bn)/TiO 2 was performed by UV-vis spectroscopy.Several adsorption parameters, for example, adsorbent dose of SA-g-p(AM-Bn)/TiO 2 , equilibrium time, initial pH, and temperature at the best optimum conditions, Spectrophotometric analysis was carried out through UV-visible spectra with the help of a double-beam spectrophotometer at 495 nm, and deionized water was used as a reference solvent.For a specific adsorption of CR dye, 0.05 g of SA-g-p(AM-Bn)/TiO 2 was utilized in 100 mL of CR dye solution at 100 mg/L for 1 h.The solution was shaken at 200 rpm in a shaker water bath.Maximum adsorption capacity Qe (mg/g) and removal percentage E% were calculated by using Eqs.( 1) and ( 2) where Qe is the amount adsorbed at equilibrium (mg/g), Co and Ce are the initial and equilibrium liquid phase concentrations of CR dye (mg/L), V is the volume of CR dye solution (L), and W is the mass of adsorbent used (g).

Recyclability study
Adsorption-desorption analysis was utilized to estimate the reusability of SA-g-p(AM-Bn)/TiO 2 .The CRadsorbed SA-g-p(AM-Bn)/TiO 2 was recyclable and regenerated via washing at various concentrations (0.01-0.1 N) of NaOH, H 2 SO 4 , HCl, H 3 PO 4 , HNO 3 , acetone, ethanol, and water.The collected SA-g-p(AM-Bn)/TiO 2 adsorbent was again rinsed via deionized water and dried at 65 °C for 12 h.After that, SA-g-p(AM-Bn)/TiO 2 was again utilized for the removal of CR dye.

Absorbency and swelling in water
The synthesized biopolymer nanocomposite's water absorbency and swelling properties were examined at 25 °C.The dry mass of the sample was recorded.The sample was then immersed in deionized water to reach equilibrium swelling.The swollen sample was removed, filtered using a clamp to eliminate excess water on the surface, and weighed.The swelling ratio (%SR) was calculated using Eq. ( 3): where M 1 is the dry mass before swelling, and M 2 is the mass after equilibrium swelling in water.This analysis provided insights into the water absorbency characteristics of the synthesized superabsorbent biopolymer nanocomposite.

Characterization for adsorbent/adsorbate
The thermalgravimetric analysis (TGA) of the SA-g-p(AM-Bn)/TiO 2 nanocomposite was studied.TGA curves of the nanocomposite obtained at a rate of heating of 50 °C/min up to 600 °C under a dry N2 flow appear (Fig. 2); one can see that the degradation method is different.It is well known that any weight loss below 200 °C is due to the loss of water unbound, while the loss in the range of 200 to 600 °C is mainly due to the degradation of organic matter.By analyzing the TGA of nanocomposite, it is quite clear that the incorporation of clay and TiO 2 NPs has an approving effect on the thermal stability of nanocomposite SA-supported bentonite 25,36 .This means that clay created a resistant path through the nanocomposite matrix to retard the decomposition process.Similarly, we can detect an improvement in thermal stability attributed to the loading of the TiO 2 duo via the presence of inorganic and organic materials at their surface.that cause bonds between the Ti and COO groups of the biopolymer.The size reduction and area increase of TiO 2 NPs on the nanocomposite assure good interactivity among them, thus making complexes more stable 13,25,36,37 .
The Fourier-Transform Infrared Spectroscopy FTIR spectra were shown in Fig. 3, as we see (A) bentonite clay, (B) sodium alginate-grafted polyacrylamide/bentonite clay (SA-g-PAM/BC), (C) SA-g-PAM/BC/TiO 2 NPs nanocomposite, and (D) SA-g-PAM/BC/TiO 2 NPs after Congo red (CR) dye adsorption 36,38 .The broad band at 3435 cm -1 and 2927 cm -1 corresponds to O-H stretching and C-H stretching vibrations.For the nanocomposite (spectrum C), characteristic peaks are observed at 3300 cm -1 (O-H stretching), 1600 cm -1 (C=O stretching), and 1409 cm -1 (symmetric COO-stretching).Bentonite clay (spectrum A) shows typical bands at 1009 cm -1 (Si-O-Si stretching), 918 cm -1 (Al-OH vibration), and 450 cm -1 (Si-O bending).Additional peaks at 1415 cm -1 (Ti-O-Ti vibration) and 450-600 cm -1 (Ti-O bending) confirm TiO 2 NP impregnation in the nanocomposite 17,27 .FE-SEM/EDX analysis was conducted to examine the morphology of the surfaces, porous structure, and properties of elemental nanocompositions.Figure 4a-d shows the SEM images along with the EDX analysis.(a) Bn clay; (b) SA-g-p(AM-Bn), (c)SA-g-p(AM-Bn)/TiO 2 nanocomposite and (d) SA-g-p(AM-Bn)/TiO 2 nanocomposite before and after adsorption of CR dye.Bentonite clay was found to have a homogeneous and smooth surface with no irregularities (Fig. 4a).It can be seen from Fig. 4b that the surface morphology appeared to be smooth with visible cavities.The SA-g-p(AM-Bn) hydrogel smoothness is a vital parameter that affects the adsorption capacity since it leads to increased hydrophilicity groups.Furthermore, the swelling of hydrogel can be contributed to by the addition of AM.In fact, AM is a molecule that can form 3D networks easily in the hydrogel.Besides, AM has a precise degree of crystallinity, which clarifies the uneven particles on the hydrogel 39,40 .
The globular particles of several sizes embedded on the surface of the nanocomposite suggest that TiO 2 NPs were successfully combined into the molecular structure of SA.Thus, the morphology of the surface SA-g-p(AM-Bn)/TiO 2 nanocomposite was a rough, irregular, and heterogonous surface that was well distributed across the nanocomposite due to the loading of TiO 2 NPs into the nanocomposite polymeric matrix.The EDX analysis confirms.The presence of O, C, Si, Ca, Mg, and Al and Ti elements in the polymeric matrix of SA-g-p(AM-Bn)/TiO 2 .From EDX analysis, the presence of the Ti element with other present elements of nanocomposite reconfirmed the successful incorporation of TiO 2 NPs in the polymeric matrix of SA-g-p(AM-Bn)/TiO 2 as shown in Fig. 4c 41 .
In line with the CR dye molecules adsorbed on the SA-g-p(AM-Bn)/TiO 2 surface, the SA-g-p(AM-Bn)/TiO 2 surface after adsorption of CR (Fig. 4d) appeared to be more compact with fewer cavities.In addition, increasing elements C and O in the EDX analysis, which belong to CR, reaffirmed the CR dye's molecules being adsorbed on the surface nanocomposite 42,43 .The transmission electron microscopy (TEM) image shown in Fig. 5.This figure shows the TiO 2 NPs embedded within the hydrogel matrix.It can be seen that SA-g-p(AM-Bn)/TiO 2 appears as regular balls along with some patchy black shapes and tends to form chain-like totals at 80 nm.Moreover, the surface of the SA-g-p(AM-Bn)/ TiO 2 is covered via a transparent layer, where TiO 2 NPs were observed embedded inside the SA-g-p(AM-Bn)/ TiO 2 and TiO 2 NPs play a pivotal role in improving stability and increasing surface area as a requisite constituent of synthesizing eco-friendly nanocomposite 2,25,37 .
The XRD patterns in Fig. 6 display characteristic peaks corresponding to (a) TiO 2 NPs, (b) bentonite clay, and (c) the SA-g-p(AM-Bn)/TiO 2 .Minor variations in clay peak intensities may arise from interactions at the TiO 2 -clay interface.Additionally, strong diffraction peaks at 2θ = 25°, 37°, 48°, 54°, 55°, and 63° match standard TiO 2 NP signals, confirming the presence of nanocrystalline TiO 2 phases 25,44 .The observed peaks at 25°, 37°, 48°, 37.97°, 48°, and 54° in the nanocomposite verify the successful integration of TiO 2 NPs within the biopolymer matrix.While the intensities are somewhat reduced and peak widths slightly broader compared to pristine TiO 2 due to the amorphous hydrogel coating, the retention of crystalline TiO 2 signatures demonstrates that the TiO 2 nanostructures are intact but interacting closely with the polymer network.Thus, XRD analysis verifies the composite nature of the synthesized nanomaterial, which contains crystalline TiO 2 nanoparticles interspersed in the hydrogel matrix 13,27,42,45 .

Effect of pH
The pH value of the solution affects the adsorption process, depending on the nature of the dyes as well as that of the nanocomposite particles.SA-g-p(AM-Bn)/TiO 2 particles exhibit high hydrophilicity due to the incorporation of monomers and clay.The hydrogel groups and carboxylic acid groups of nanocomposite particles are protonated/deprotonated with the pH change of the solution and induce hydrophilicity / hydrophobicity in the hydrogel nanocomposite network.The relations between the removal percentage E% of CR and solution pH are shown in Fig. 7.The removal percentage of CR was increased with an increase in solution pH up to 7 46,47 .After that specific pH value, the removal percentage of CR was increased up to 11.This trend was observed due to nature's anionic CR as well as the presence of hydrogel groups or carboxyl acid groups in nanocomposite particles.
In an acidic medium, all hydrogel groups or carboxylic acid groups of nanocomposite were totally deprotonated, and negative charges were induced in the network of hydrogels due to the deprotonation of these groups.The presence of a negative charge inside the nanocomposite disables the uptake of anionic CR dye from an aqueous solution due to its low electrostatic attraction.Thus, a decrease in the removal percentage of CR was observed with a decrease in the solution pH at 4. At low pH, water was expelled out and the sieve size of the nanocomposite was decreased, which also resulted in a decreased uptake of CR molecules.Also at low pH, negatively charged carboxylate groups of nanocomposite particles and anionic CR dye molecules repel each other due to the same charge due to a decrease in the removal percentage of CR.Thus, the best optimum value of pH for the best removal percentage (CR) is at pH 7 48,49 .

Effect of adsorbent dosage
The influence of the adsorbent dose of SA-g-p(AM-Bn)/TiO 2 nanocomposite on CR adsorption was examined in the range of different doses (0.02-10.1 g) at 30 °C, pH 7, and 100 mg/L concentration of CR dye (Fig. 8).Increased weight of SA-g-p(AM-Bn)/TiO 2 results in increased percentage removal of CR dye; the percentage removal was increased from 57.5 to 98.9% for 0.02 g and 0.1 g, respectively.Due to 0.05 g/L of SA-g-p(AM-Bn)/TiO 2 weight resulted in 92.12% CR dye absorption.The enhancement in removal efficiency with increasing adsorbent weight could be explained by the presence of additional adsorption sites on the adsorbent.Because of the strong competition between the adsorbent and active sites on the adsorbent.An increase in the percentage of dye removal with adsorbent weight was related to increases in the adsorbent surface areas, improving the number of active sites obtainable for adsorption, as reported already in other cases 50,51 .The increase in removal capacity of CR with nanocomposite is due to the introduction of more binding sites for adsorption.The initial parameter explaining this characteristic is that adsorption sites remain unsaturated through the adsorption process, whereas the number of active sites obtainable for adsorption sites increases via increasing the weight of nanocomposite 34,47,51 .

Effect of temperatures and thermodynamic parameters
To determine whether the ongoing adsorption method was exothermic or endothermic.The adsorption thermodynamics were determined for dye-adsorbent systems.The thermodynamic parameters, including changes in Gibbs free energy (ΔG), enthalpy (ΔH), and entropy (ΔS), were calculated to evaluate the spontaneity and heat aspects of Congo redCr dye adsorption onto SA-g-p(AM-Bn)/TiO 2 nanocomposite 12,43 .Batch experiments were performed at varying temperatures (10-40 °C) and concentrations of CR dye (10-100 mg/L) to analyze the effect on adsorption capacity, as shown in Fig. 9.The results indicate that the adsorption capacity (Qe mg/g) of CR dye increased while increasing the solution temperature for all CR dye concentrations.The adsorbent's efficiency of adsorption changes with temperature.Thus, the temperature parameter is important as a physicochemical process.An adsorption endothermic process involves a directly proportional adsorption increase with temperature, caused by an increase in adsorption active sites and the dye molecule's mobility with increasing temperatures.It was found that the increasing temperature of the solution causes a decrease in aqueous phase viscous force resistance, thereby leading to the dye molecule's faster diffusion across the adsorbent particles' external boundary as well as internal pores.The removal process was also significantly affected by the change in adsorbate molecules' solubility in some cases.At high temperatures, pore size enlargement also causes increased adsorption 1,43,[52][53][54] .Determined of the thermodynamic parameters including (∆H), (∆G), and (∆S) of the adsorption process.The equilibrium constant (K e ) of the adsorption at each temperature, was calculate via Eqs.( 4): The free change energy of adsorption were calculated assuming an activity coefficient of unity for low solute concentrations (Henry's law) by Eq. ( 5): where G : Gibbs free energy (J.K -1 .mol. - ), R is the gas constant (8.314J.K -1 .mole - ), T is the absolute tempera- ture in Kelvin.
The enthalpy change of adsorption may be obtained from the following Eq.( 6): (4) www.nature.com/scientificreports/Calculated, the values of all of the thermodynamic parameters at different temperatures appear in Table 1.The positive values of ΔH and ΔS indicate an endothermic process with increased randomness at the adsorbentadsorbate interface, whereas the negative ΔG values predict the spontaneous nature of the CR dye adsorption onto SA-g-p(AM-Bn)/TiO 2 .The decrease in Gibbs free energy with increasing solution temperature also indicates that adsorption is favorable at high temperatures.As temperature rises, the increasingly negative ΔG values predict greater thermodynamic favorability of adsorption at higher temperatures 14,55,56 .

Recyclability and reuse study
Evaluating the regenerability and reusability of adsorbents is crucial for viable industrial applications from both economic and environmental standpoints.Desorption studies help elucidate dye removal mechanisms and optimal regeneration strategies for recycling spent adsorbents, lowering operating costs, and mitigating secondary waste pollution.Desorption experiments were performed on dye-loaded nanocomposite using various concentrations (0.01-0.1 N) of NaOH, H 2 SO 4 , HCl, H 3 PO4, HNO 3 , acetone, ethanol, and water as eluents.
The SA-g-p(AM-Bn)/TiO 2 nanocomposite and SA-g-p(AM-Bn) hydrogel matrix were tested for CR dye removal over six repeated adsorption-desorption cycles under optimal conditions (Fig. 10).The adsorption efficiency of the SA-g-p(AM-Bn)/TiO 2 nanocomposite remains constant through six cycles.In contrast, after four cycles, the SA-g-p(AM-Bn) hydrogel showed a slightly declining performance 57,58 .
The choice of this procedure is associated with the fact that a new equilibrium between the adsorbent and the dye will be established following the formation of new species in water (natural medium pH = 7).The role of water is to increase the CR dye solubility for their interactions on the adsorbent nanocomposite surface.Also, water has a hydrophilic (OH) functional group.This group can firmly adsorb onto the adsorbent nanocomposite surface and interact with the functional group of the nanocomposite 59 .This helps the water molecules firmly adsorb on the adsorbent nanocomposite.In addition, it is also possible that the addition of water may have decreased the interaction extent between the adsorbent nanocomposite surface and CR dye molecules; as a result, CR dye molecules get desorbed from the nanocomposite surface.During the first cycle of adsorption-desorption, regeneration of nanocomposite and hydrogel was effective in desorbing CR (92.81-85.985)and (82.61-68.98%),respectively, after 6 cycles.A possible decrease in percent adsorption after every cycle might be due to the blockage of some active sites by CR dye that are difficult to desorb because of their strong chemical interactions with the nanocomposite surface 60,61 .

Comparative adsorption study
The CR dye removal efficiencies of TiO 2 NPs, SA-g-p(AM-Bn) hydrogel, and the SA-g-p(AM-Bn)/TiO 2 nanocomposite adsorbents were compared via batch experiments.0.05 g doses of each adsorbent were added to 100 mL solutions of 100 mg/L CR dye concentration and agitated for 1 h.The remaining dye concentrations (6)   in the separated supernatants were analyzed by UV-vis spectrophotometry at the maximum absorbance wavelength.As shown in Fig. 11, the nanocomposite exhibits the highest CR removal percentage, outperforming the individual TiO 2 nanoparticles and hydrogel matrix 25 .
where qt and q e are the adsorbate uptake at time t (mg/g) and at equilibrium (mg/g), k f and k 2 are the rate constants for pseudo-first order (min −1 ) and pseudo-second order (g/(mg•min)) models, α is the initial adsorption rate (mg/(g•min)) and β is the desorption constant (g/mg) in the Elovich model.The kinetic model fits are displayed in Fig. 12, and the parameters are listed in Table 3.The higher correlation coefficients (R 2 > 0.99) indicate excellent agreement of experimental data with the pseudo-First order model, suggesting chemisorption governs the rate-limiting step for CR dye uptake.Pseudo-second order and Elovich models showed poorer fits 1,11,14,55 .

Adsorption isotherms
The Langmuir and Freundlich isotherm models were applied to analyze the equilibrium data further.These popular nonlinear isotherm models are described by Eqs. ( 10) and ( 11): where qe is the amount adsorbed at equilibrium (mg/g), Ce is the equilibrium concentration (mg/L), and qm and K L are the Langmuir maximum adsorption capacity (mg/g) and affinity constant (L/mg), respectively.K F (mg1 −1 /nL1/ng −1 ) and 1/n are the Freundlich constant and heterogeneity factor.
Figure 13 displays the nonlinear isotherm fits, and Table 4 compiles the model parameters.The excellent correlation coefficient (R 2 > 0.999) suggests the Freundlich isotherm better represents CR dye binding onto the nanocomposite surface.The 1/n value of 0.683 indicates favorable physicochemical adsorption.The high K F value (44.368) further supports substantial adsorption capacity.While the Langmuir model showed poorer agreement, the maximum monolayer coverage from this approach was calculated as 180.2 mg/g 1,9,64-66 .

Suggested adsorption mechanism
The proposed adsorption mechanism of CR dye via SA-g-p(AM-Bn)/TiO 2 nanocomposite hydrogel has several kinds of interactions, as shown in Fig. 14.There are several functional groups available on nanocomposite (10)    www.nature.com/scientificreports/surfaces that can adsorb CR dye.These functional groups come from the natural bentonite clay, sodium alginate biopolymer, and titanium dioxide.Thus, the active groups are (-OH), (C=O), (OH 2 + ), (-NH 2 ), (≡Si-OH), (≡Al-OH) and (TiOH 2 + ), on the surface of nanocomposite.This mechanism involves the electrostatic attraction among negatively charged of (SO 3 -) sulfonate groups of CR dye and hydroxyl groups (-OH) and amino groups (-NH 2 ) 30 .
Adsorption mechanisms can also contain two kinds of H-bonding, for example, dipole-dipole hydrogen bonding and Yoshida H-bonding.The H-bonding dipole-dipole interactions among free hydrogen of nanocomposite with oxygen and nitrogen in the structure of CR dye, while Yoshida H-bending occurs among the aromatic ring of CR dye and the OH on the nanocomposite surface.Finally, n-π interaction occurs among electron-donating groups of nitrogen and oxygen on the nanocomposite surface and π-system in the aromatic ring of CR dye 67 .

Conclusion
The synthesized SA-g-p(AM-Bn)/TiO 2 nanocomposite hydrogel was confirmed via the results of XRD, TGA and EDX.The FESEM micrographs depicted the highly rough and granularly spongy surface of the nanocomposite, thus being highly favorable to the removal of toxic CR dye.The 92.9% was the highest reported adsorption of CR dye on SA-g-p(AM-Bn)/TiO 2 nanocomposite at optimized conditions (100 mL of 100 mg/L of solution CR dye, SA-g-p(AM-Bn)/TiO 2 nanocomposite adsorbent dose = 0.05 g, time = 60 min, pH = 7).The best adsorption capacity was 185.92 mg.g -1 , and CR dye adsorption fitted better with the Freundlich isotherm and the pseudosecond-order model.Moreover, recyclability of SA-g-p(AM-Bn)/TiO 2 was performed and exhibited 82.2% CR dye adsorption even after six successive adsorption-desorption cycles.Hence, re-cyclable SA-g-p(AM-Bn)/TiO 2 indicates CR dye adsorption and can be used as an excellent adsorbent in textiles' dyes.Table 4. Several factor isotherms for the adsorption study of CR on to SA-g-p(AM-Bn)/TiO 2 .

SA-g-p(AM-Bn)/TiO 2
Freundlich Q e = K f C

Figure 11 .
Figure 11.Adsorption of comparative between several surfaces for removal percentage on CR dye.

Table 2 .
Comparative of desorption removal efficiency of different kind solution for the CR dye on to SA-gp(AM-Bn)/TiO 2 .

Table 3 .
First model, second model, and Elovich model including correlation coefficients for CR adsorption on to biopolymer.